Surging throttle and lack of power

[ctm4003@bvu.net]

I have a 1951 Cub. Prior to carb rebuild, would only run under full choke. Rebuilt carburetor and cleaned it out thoroughly. Now when it cranks, throttle surges then settles down. Not immediately responsive to throttle change. Seems to have no power under load. Can only drive in 2nd and 3rd if driving down a grade.Basically, no power under load.

[Glen]

Hi,
Since you were repairing the carburetor, here is info for the IH carb, you can look through it and see if you did all it says.
The float level in the carb needs setting right, or the engine may not run right.
The main jet needs cleaning on the end with a small wire, if plugged fully or some, the engine won't run right.
You didn't say if it has the IH carb. The IH carb would have come on a 1951 Cub.
Some people change them to a Zenith carb, which is different.

The Cub may need a good tune up, and the ignition timing set using a timing light, like cars with a distributor used. If the timing is off, the engine can have less power than it should.

Here is some info for cleaning and repairing the IH carburetor, if you need it.
You might not need all the info, but it's important to set the float level right.

Before taking the carb apart, clean the outside of it, to keep dirt out of the carb.
When taking it apart, pull the 2 halves of the carb straight apart several inches, there is a small idle tube that screws into the top half of the carb, and sticks down into the bottom half of the carb, it can break if you twist it, or don't pull the 2 halves straight apart.

Use carb cleaner, or solvent to clean inside the carb.
All the passages need to be open and clean.
Blow them out with compressed air, and be sure they are open. Use your safety goggles when using the air.
Remove the main jet, the 6 sided brass colored part low on the side of the carb, and run a small wire in the hole in the end of it, to be sure it is open, and then wash the jet.
Tighten it gently, it can break. It should have a fiber washer under the head, to seal it, sometimes if the washer is old, it won't seal if you reuse it.

The needle and seat could have dirt in it, making it not seal. Remove the float and the needle and check for dirt, clean the needle and seat.
Check the float, it should look in good condition, not collapsed or corroded.
Shake it gently while it is off, it has to be dry inside, no gas inside it.
If it is good, it can be used again.
Assemble the needle and float again.

The tab on the float arm has to face up.
Sometimes the float is upside down.
Put it in the right way, if it is upside down.

It is important that the float level and float drop be set exactly.
People on here have said that their carb wouldn't work right until they set the float level exactly.

Below is a page from the Cub and LoBoy service manual showing setting the float height.
The IH carburetor is in the upper pics on the page, the Zenith carb in the lower pics.
To set the float level, turn the top half of the carb upside down, like the pic of the Zenith carb, in the lower right of the page.
Don't use the float level specs shown for the Zenith carb, for an IH carb, they are different.
The gasket between the halves has to be off, when measuring the float level of the IH carb.

http://www.farmallcub.info/manuals/gss- ... 002-09.jpg

Bob McCarty converted the measurements to use a dial caliper, it is easier.
The float level is 1.4"
The float drop is 1.6"

Bend the float arm between the float and where the needle touches it, to change the float height.

Turn the top half right side up to set the float drop.
Bend the tab in or out to change the float drop.

The needle and seat could be wornout from years of use, and don't seal, then the carb can drip on the ground when the Cub is sitting.
I have seen them that were wornout, they dripped, and then it quit dripping after putting in new ones.
The IH carb has a small screen where the gas line attaches to the carb, be sure it is clean.
Tighten everything on the carb gently, the IH carb is soft metal, and threads can strip.
The carb top can warp if the screws are tightened too much.
You could check the gasket surface of the top when it is off using a straight edge.
TM Tractor, at the bottom of the page, has new carb parts.

Below are pages from the Cub parts manual showing the IH carb.
The main jet is number 28 in the pic.

http://www.farmallcub.info/manuals/cub_ ... 012-12.jpg

http://www.farmallcub.info/manuals/cub_ ... 012-13.jpg

Cubs have a manual choke, you have to use the right amount of choke when the engine is cold, until it warms up, so it runs good.

The 1st pic below from TM Tractor is an IH style carb, it is a replacement carb, I think, but looks like the IH carb.
The 2nd pic is an IH carb float from TM Tractor, showing the tab for the float drop.
The 3rd pic shows cleaning the main jet.

[inairam]

Other than needing full choke did the cub run ok before the carb was off?

If so two things I would try first.

1) make sure there are no air leaks at the carb/manifold connection. Some people spay starter fluid at it to see if there is a change in rpm or you can tighten the bolts a very little bit. do not want to break the carb flange.

2) I know it is the same carb and all but redo the linkage adjustment from governor to carb per the manual. It seems more times than not when I take a carb off I have to redo that adjustment.

[ctm4003@bvu.net]

Thanks Glenn. I did all that on the carburetor. Soaked it in carb cleaner, blew it out, cleaned out jets with wire, rebuilt carb totally, set float height per the reference you provided. It starts on first try. Just seems like there is a lag in reving once throttle is moved.

[ctm4003@bvu.net]

What do you mean by linkage adjustment?

[Don McCombs]

[Eugene]

After adjusting the governor to carburetor linkage, check the ignition/ignition timing.

[PFord]

When my Cub had this issue, turned out to be 2 things:

1. Warped top of the carburetor letting in unmetered air.
2. Piece of rust lodged in between the tank and sediment bowl. Had to take the sediment bowl assembly off the fuel tank to find it.